(a) Field of the Invention
The present invention relates to a low-impedance decoupling device and, more particularly, to a low-impedance decoupling device implemented as a line device and suitably used for decoupling an electromagnetic noise wave in a high-frequency LSI.
(b) Description of the Related Art
Electromagnetic interference is caused by an electromagnetic wave induced by a switching operation of a semiconductor element used as a switching device in a digital circuit. The electromagnetic wave includes high-harmonic wave components of a clock frequency, or fundamental wave, which cause high-frequency noise in the semiconductor circuit. Some of the high-frequency electromagnetic wave components generated by the switching device in the LSI are coupled with the signal lines etc. in the LSI, package or printed circuit board (PCB) by induction coupling while the electromagnetic components transfer along a power-source distribution line in the LSI. The induction coupling of a high-frequency electromagnetic wave with the signal line causes emission of an electromagnetic wave from a signal cable or apparatus and thus leakage thereof toward outside the LSI.
If a surge impedance, i.e., an impedance of the power-source distribution line in the high-frequency range as viewed from the switching device in the LSI is higher, the high-frequency electromagnetic wave generated by the switching device in the LSI interferes with the signal line to generate a distortion in the signal voltage thereon. For suppressing such an interference, it is effective to insert a power-source decoupling circuit in the power-source distribution line.
The conventional decoupling devices for semiconductor circuits, as in the case of the “Semiconductor die having an on-die decoupling capacitance” described in Patent Publication JP-A-10-270643, have smaller dimensions compared to the wavelength corresponding to the operational frequency of the semiconductor circuit. Thus, it is general to add a decoupling device including a capacitor, construed as having a lumped-parameter capacitance, to the power-source distribution line as a low-impedance device.
It is to be noted that the insulator film underlying the power-source distribution line in the LSI is generally implemented by a field oxide film (element-isolation oxide film), and has a thickness of around 500 to 1000 nm (5000 to 10000 angstroms).
When a capacitor 21 is used as the decoupling device for a power source distribution line 20 by connecting the capacitor 21 thereto, as shown in FIG. 13A, a serial inductance of the connection line 22 must be considered. That is, the inductance of the connection line 22 degrades the decoupling performance of the capacitor 21.
It may be considered to separate the capacitor into a plurality of small-capacitance capacitors and dispose the same separately in the LSI, package and PCB. This effectively reduces the inductance of the connection line 22 for the capacitor 21 to suppress the degradation of the decoupling performance of the capacitor 21.
However, the inductance of the connection line 22 cannot be neglected even if the capacitor 21 is separated into a large number of small-capacitance capacitors. For example, if such a capacitor 21 is located adjacent to the distribution line 20 without a gap therebetween, as shown in FIG. 13B, the mean distance between the distribution line 20 and all the portions of the capacitor 21 is half the width of the capacitor 21, thereby degrading the decoupling performance of the capacitor 21 corresponding to the distance of the half width.
In addition, since the capacitor has a dominant inductance component at a frequency above the series resonance frequency of the capacitance and the inductance of the capacitor 21, a higher frequency degrades the impedance characteristic of the capacitor 21 accordingly.
In short, since the capacitor itself assumes an inductance property in a frequency range above several hundreds of megahertz (MHz) even if the capacitor is separated into a large number of small-capacitance capacitors and provided separately, the capacitor does not afford a capacitive decoupling device in the high-frequency range.
In the circumstances wherein the current digital circuit has a higher operational frequency as high as gigahertz order, the capacitor should have a lower impedance required of the decoupling circuit in the frequency range above the several hundreds of megahertz.
As described above, the capacitor generally used as the conventional decoupling device assumes an inductance property in the frequency range above the several hundreds of MHz. Thus, it is desired to provide a low-impedance device or a low-impedance structure for achieving a decoupling circuit effectively performing the decoupling function in a digital circuit having a clock frequency of a gigahertz order.